Note that I am doing the PW math purely on load sifting. We have a 400A feed with a backup industrial generator (which can generate power cheaper than PG&E's peak rate), so I don't need the powerwalls for emergency power, and I don't want to redo my panels to try and have some circuits backed up and others not. The solar equipment is hooked to the utility side of the Genset ATS, so it would be easy to put the Gateway in between the main panel and the Solar panel, if I just wanted load shifting.
I assume it's possible to configure the PW to do this sort of load shifting behavior.
Am I missing something?
thx
mike
Ok having reread your initial post, I see the reason you don't need backup power as you already have it. I re-iterate that you are going to overspend on backup power you say you do not need with the Tesla PW system.
However your math doesn't work out in my eyes because if you still have a bill that means you are a net consumer. If you were a net generator you might get a small check at your true up bill, and that would credit you with a tiny $0.03 per kWh.
Since you are a net consumer under NEM 2.0 your PV credits are considered at the same rate you would buy them at the time they were generated. PV power at noon generated under EV2A is worth about $0.25 per kWh as a bill offset. PV generated at 4 pm is even more valuable, though most PV systems are kicking down at this time.
The majority of the year, the best cost shift you can get is $0.18/kWh. During 4 months of the summer, you can get $0.30 delta between the lowest and highest priced power. Also Powerwalls are about 90% round trip efficient, so you need to count that into your math. My rough math says you would make less than $4 per day on the time shift of 24 kWh from off peak to peak prices.
As to the cost of the generator power, you are only looking a the fuel, not the upkeep cost per hour of runtime. Also there are some societal costs if everyone decided to fire up generators at peak power prices.
With a 400A feed and an industrial generator, you need to find a place to connect where you have a 200A subfeed, since the Gateway has this limitation.
Exactly - the solar would drop offline during an outage, just like it does now.
I have a diesel genset that works great for backup, doesn't depend on nat gas lines working after an earthquake, and has a 400A transfer switch that backs everything up without a fuss. The house is very efficient (all LED lights, heat pumps for AC and heating, etc...), so just a 30KW genset is more than enough to keep the house running with lots of margin, and with a 200 gal tank, it can run a long time even in bad weather without refueling.
My perfect world would have the PW's integrated with the Genset and be able to be charged by the genset and solar, and allow the generator to be run at the most efficient load, about 50% of capacity, and use solar as much as possible and the genset when solar can't handle it. But this is not possible with the way Tesla has engineered things, so my case is just on reducing the PGE burn.
So yes, the solar would not be usable during an outage, which is the way it is right now. Is this mode supported by the software in the system? There are no loads on the panel with the inverters and where the PWs would be installed.
If your load never exceeds 30 kW you could power your whole house on a 200A grid feed, then you could put the Tesla gateway in between the MSP and the ATS. You would have a first backup of the PV+PW and the second backup of the generator if the batteries were depleted.
To do this you either need more batteries in the backup side, or less PV in the backup side. It is possible to split the PV into 2 different systems, where 10-15 kW max of PV is on the backup side and the rest is not on the backup side. This may not be easy with your inverter and battery loations compared with the MSP.
For the size of your home and bill I'd recommend 4-5 power walls and hopefully never fire that stinky diesel generator again. Take the SGIP and the Tax credit to offset the cost and find some personal value in backup power that doesn't require combustion.
Depending on the age of the PV system, the powerwall backup system might play somewhat nice with an overabundance of PV, or it might snap on then off every 5 minutes if it's older equipment than about 3 years ago. Even newer equipment though would put your house through constant frequency shifting while on backup battery power, which isn't really ideal, but might not be a deal killer for all homes.
There is no Tesla-approved way to integrate your generator as a power source that charges the ESS, as long as your home is grid connected. Other products do offer this generator charging feature.
Again, I gave up on using the powerwalls for ANY backup. That's why my business case is just for load shifting, and they would be wired on the line side of the generator transfer switch, and with NO loads of any kind behind the gateway. So there is no interaction with the generator at all - effectively the solar and powerwalls are all tied to PGE, and the house loads and genset are completely isolated from them except at the main panel where the utility feed connects to the transfer switch and to the GW.
The powerwalls don't need to be stable during a power outage. If I needed to, I guess I could take one of the inverters and put it on another panel (at some expense), but can't I just tell the powerwall to drop offline during an outage? No need to charge during the outage. The gateway should see no voltage on the line side and isoilate everything behind it. It's not a real transfer switch like you would use with a generator - it just cuts off the grid right?
On the pricing, I am not assuming any export to PGE from the powerwalls (I am on NEM 2.0). The idea is to shift the excess power from solar during off-peak times to the PWs, which would then offset peak hour consumption from the house. PG&E is not paying me at the retail rates I am pretty sure based on my bill. So if I run the meter backward during off peak times, and take THAT excess power and store it in the PW, and then use it to null out any peak hour consumption from the house, the delta should be my peak hour pricing minus what PG&E would have paid me, correct? You are definitely right that peak hour pricing in the winter is 0.43 during the winter and not 0.51. But you are also right that summer peaks are now 0.55/kWH - yet another rate increase to deal with.
So assuming I would normally consume 24 kWH in the peak times or more per day, then I can use the stored power to offset that right? And I am just thinking about the surplus energy during off peak times, though I could also include partially peak times as well, but that would require a larger amount of storage, and I am not sure I can consume all that at night anyway.
This is an interesting idea, and does give what you want, but feels like a square peg in a round hole.
If you went with this approach, you might want to simply turn off the Powerwalls entirely under grid outage. There are a couple of ways to do this with the remote battery disconnect button. I have never heard of a customer not backing up any loads on any Powerwall system and we have installed a few thousand units. I suspect when you re-do the math you will see that the payback is much longer than you initially thought.
